The present invention relates generally to torque converters, and particularly to a single stage, twin valve, variable torque converter, capable of converting a constant speed input to a variable, forward direction output torque and providing a low reverse direction output torque.
In machines such as heavy lift cranes and crane-type excavators (draglines), it is customary to use a constant speed engine to drive a hoist cable drum for lifting the crane's load. In crane-type excavators, it is also customary to use the same constant speed engine as well to provide power to a dragline drum for pulling the dragline attached to the crane's bucket during the excavation operation itself. Three stage, variable torque converters, such as that shown in Van De Hey, U.S. Pat. No. 3,335,568, assigned to the assignee of the present application, have been used for a number of years to convert the engine's constant speed input to a variable torque output to power the hoist cable drum and the dragline cable drum in cranes and crane-type excavators manufactured and sold by the assignee of the present invention.
In variable torque converters such as described in the Van De Hey patent, the output torque is varied by means of a single valve in the form of a cylindrical sleeve which slides axially between the vanes of the input impeller and the first stage (set of vanes) of the three stage output turbine and regulates fluid flow from the input impeller to the three stage output turbine. By controlling the flow of fluid between the impeller and the turbine, variable forward output torque is provided from a constant speed input to power the hoist cable drum and dragline cable drum of heavy lift cranes and crane-type excavators. The Van De Hey-type three stage, variable torque converter has in large part eliminated the necessity of providing mechanical friction clutches which are prone to wear and early failure in heavy lift cranes and crane-type excavators.
In crane-type excavators particularly, efficiency is achieved not only by rapid loading and hoisting of the loaded bucket, but also by rapid lowering of the empty bucket and paying out of the dragline. In order to lower the empty bucket rapidly and pay out the dragline rapidly and thus insure a rapid duty cycle, it is necessary for the hoist cable drum and dragline drum to rotate in the reverse direction and thus drive their respective torque converter output shafts and turbines in the reverse direction. As the reverse direction speed of each torque converter's output turbine increases, a reverse output torque is generated even though the control valve in the Van De Hey-type converter is closed so that the oil flow between the input impeller and the output turbine is greatly restricted.
As a result of the reverse output torque, it has been necessary in the past to provide auxiliary hydrostatic motors at the output shaft of each of the torque converters in order to overcome the reverse torque and drive the torque converters in the reverse direction to provide a "power down" capability to insure the rapid descent of the excavator bucket and to pay out the dragline to assure that the bucket can be properly placed in the excavation area.
Overcoming the reverse output torque represents energy wasted in the form of heat generated in the torque converter. Any reduction in the reverse torque characteristics of the torque converter translates into savings of energy. Moreover, if the reverse torque can be lowered, the size and weight of the auxiliary hydrostatic motors can be decreased, and perhaps the "power down" hydrostatic motor can be eliminated altogether.